/Engine/Source/Editor/EditorFramework/Public/Toolkits/IToolkitHost.h
Gets a widget that can be used to parent a modal window or pop-up to. You shouldn't be using this widget for anything other than parenting, as the type of widget and behavior/lifespan is completely up to the host.
[TSharedRef](API\Runtime\Core\Templates\TSharedRef)< class [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > GetParentWidget()